The present invention relates to a force-measuring apparatus, particularly a weighing cell, with a load receiver serving to receive the force that is to be measured. The load receiver is restrained to a mode of parallel motion in relation to a stationary part by two guides that extend in parallel planes and are rigid in all directions within the planes but movable in the direction transverse to the planes. Each of the guides is connected at one end to the load receiver and at the opposite end to the stationary part. The apparatus includes an adjuster device by which the position of at least one of the attachment areas where one of the guides is attached to the stationary part can be adjusted in the direction transverse to the planes of the guides. The adjuster device has an arm that extends approximately parallel to the planes of the guides and is rotatably connected to the stationary part so that the arm is rotatable about a pivotal axis that is approximately parallel to said planes and approximately perpendicular to the lengthwise direction of the arm. The arm includes the attachment area as well as an adjustment area for an adjusting member that serves to tilt the arm about its pivotal axis.
Weighing cells of this kind have the purpose of generating a position-independent measuring signal for the weight of a load that is placed on a weighing pan or load platform attached to the load receiver, i.e., the signal for a given load has the same magnitude no matter where the load is placed on the weighing pan. The parallelism of the guides is critical for the position-independent property of the cell, which requires that the weighing pan or platform move in a strictly parallel mode in response to a load. Particularly with high-precision balances, it is therefore necessary to provide an adjuster device that allows the parallel alignment of the guides in relation to each other to be precisely adjusted.
A weighing cell with an adjuster device of this kind is proposed, e.g., in DE 86 15 750.7 U1. The weighing cell described there has one end of the guides connected to the stationary part through an additional parallel-guiding linkage. The upper portion of the additional parallel-guiding linkage carries the attachment area for attaching the guides. A bottom-end surface of the additional parallel-guiding linkage and an upward-facing fixed surface of the stationary part enclose between themselves an approximately wedge-shaped gap. Inserted in this gap is a bolt that extends parallel to the planes of the guides. An adjustment screw runs through the bolt, transverse to the longitudinal axis of the latter, and is held in a threaded hole of the stationary part. The adjustment screw serves to move the bolt in or out within the wedge-shaped gap where the bolt bears against the bottom-end surface of the additional parallel-guiding linkage on one side and against the upward-facing fixed surface on the other side. This arrangement allows the upper portion of the additional parallel-guiding linkage with the attachment area, and thus the attached ends of the guides, to be adjusted up or down in the direction transverse to the guides. This adjustment serves to align the guide that is attached to the upper portion of the additional parallel-guiding linkage to be exactly parallel to the other guide. However, this adjuster device is expensive to manufacture because, in addition to the parallel-guiding system for the load receiver, an additional parallel-guiding linkage is required that carries the attachment area for attaching one of the guides to the stationary part.
In a weighing cell of the kind named at the beginning and known from DE 87 08 485.6 U1, the attachment area that is adjustable with the adjuster device and serves for the attachment of one of the guides to the stationary part is formed on an arm that is connected to the stationary part through a pivotal flexure. In the weighing cell according to the aforementioned reference, the arm is formed by a separating slit that extends into the stationary part about parallel to the plane of the guides and opens to the rear of the weighing cell. Enclosed between the dead end of the slit and a top surface of the stationary part is a narrow bridging portion of material that forms the pivotal flexure. The adjustable attachment area is arranged on the topside of the rear portion of the arm, i.e., at the opposite end from the pivotal flexure. To adjust the position of the attachment area in the transverse direction in relation to the plane of the guides, the weighing cell of the aforementioned reference has an adjusting member in the form of a differential screw. One threaded section of the differential screw is engaged in a threaded hole through the rear portion of the arm, while the other threaded portion (of different pitch) is engaged in a threaded hole of the body of the stationary part. The rear portion of the arm can be precisely adjusted up or down by turning the differential screw.
However, the small angular movement of the arm that is produced by turning the differential screw of this weighing cell also causes a slight tilting of the threaded hole in the arm in relation to the threaded hole in the body of the stationary part. Thus, by turning the differential screw, transverse forces are introduced into the arm, which can lead to an undesirable distortion of the arm. To eliminate this problem, an insert is placed in the slit of the known weighing cell. The inserted part contacts the underside of the arm and the adjacent body surface of the stationary part along straight contact lines that are parallel to each other.
Although this adjuster device which comprises the pivotally connected arm, the differential screw and the insert, as just described, has significant technical advantages over the adjuster device with the additional parallel-guiding linkage, the manufacturing process of the adjuster device is still expensive because, in addition to cutting the internal threads to receive the sections of the differential screw in the arm and the body of the stationary part, it is also necessary to manufacture and install an insert with two exactly parallel contact lines.
In view of the aforementioned problems inherent in state-of-the-art devices, it is therefore the object of the present invention to provide a force-measuring apparatus that belongs to the kind described at the beginning but offers the advantage of simplified construction while assuring a high degree of measuring accuracy.
According to the invention, this goal is accomplished through a design advancement of the known force-measuring apparatus. The essential characteristic features of the inventive design are that in the lengthwise direction of the arm, the distance from the adjustment area for the adjusting member to the attachment area is at least exactly the same as the distance from the attachment area to the pivotal axis so that, when the adjustment area is moved up or down by a given amount, e.g., through the action of a suitable adjusting member, this causes the attachment area to move by a lesser amount.
The invention is based on the observation that the arm, which is pivotally connected to the stationary part so as to provide an adjustable attachment area, can also be used as a lever to effect a reduction of the amount of displacement-travel at the ratio of the respective distances of the adjustment area and the attachment area from the pivotal axis. The lever reduction allows the position of the attachment area to be precisely adjusted through the use of a standard adjustment screw with a single fine-pitched thread. The adjustment screw is engaged in a corresponding thread in the body of the stationary part and applies its adjusting force to the adjustment area of the arm or, alternatively, the adjustment screw is engaged in a corresponding thread in the adjustment area of the arm and pushes against the body of the stationary part. Because an adjustment screw of this kind is engaged only in one mating thread, the screw can be arranged so that, when an adjustment requires the arm to be tilted, there will be no appreciable transverse forces that could cause a distortion of the arm. Consequently, the inventive design advancement of a force-measuring apparatus allows a precise adjustment of the attachment area with a standard adjustment screw that is engaged in only one mating thread, and the additional insert of the prior art is no longer required. Thus, the weighing cell according to the invention can be made in a simple manufacturing process while assuring a high level of measuring accuracy.
In a design version that was found to be particularly practical because it allows the weighing cell to be assembled from an exceptionally small number of components, the arm that carries the attachment area is formed integrally as a portion of the stationary part and is connected to the latter through a pivotal flexure in the form of a narrow material connection.
To provide a particularly compact weighing cell, it is advantageous if the arm that comprises the attachment area has the form of a one-armed lever where, in the lengthwise direction of the arm, the attachment area is arranged at a location between the adjustment area and the pivotal axis. This allows the space between the stationary ends and the load-receiver ends of the guides to be used for the arm that comprises the attachment area, if the armxe2x80x94as seen in a sectional plane perpendicular to its pivotal axisxe2x80x94extends from the pivotal axis in the direction towards the load-receiver ends of the guides. In this arrangement, the space between the pivotal axis or pivotal flexure and the attachment area can be used to accommodate the part of the guide that is to be connected to the attachment area, if the connecting flexure element is arranged so that, from its attachment to the guide, it extends forward in the direction towards the load-receiver end of the guide. With this configuration, the arm that comprises the attachment area can be accommodated almost completely within the spatial envelope that would in any case be required for the guides.
A particularly large reduction ratio of the displacement introduced by the adjusting member is achieved if the arm extends approximately over the entire length of the guides between their stationary ends and their load-receiver ends.
If the stationary part extends also approximately over the entire length of the guides, the arm with its pivotal connection to the stationary part can be realized by providing a slit in the stationary part. The slits may be open at the front surface of the stationary apart in the vicinity of the laod receiver. However, if the front surface of the stationary part near the load receiver also has a coupling area for a balance lever that transmits all or part of the weighing force from the load receiver to a measuring transducer, it has proven to the particularly practical if the arm extends between the coupling area of the balance lever and the pivotal axis of the arm, with the slit running out of a surface of the stationary part that runs parallel to the planes of the guides, e.g., the top surface of the stationary part.
To make the configuration of the inventive force-measuring apparatus as compact as possible, it has further proven to be very advantageous if the arm extends essentially in a plane that runs between and is parallel to the planes of the guides, because this configuration allows the space between the guides to be used for accommodating the arm.
The arm that comprises the attachment area can be given a particularly weight-saving configuration combined with exceptional torsional stiffness around the attachment area if a section of the arm that comprises the attachment area is thicker (in the direction transverse to the planes of the guides) than a section of the arm that comprises the adjustment area.
If the arm is designed exceptionally long in order to achieve a particularly large reduction ratio, it is possible to stabilize the arm against moving sideways and at the same time assure the desired adjustability, if the inventive force-measuring apparatus includes a stabilizer element that is connected at one end to the arm and at the other end to the body of the stationary part, so that it stiffens the arm against moving sideways in parallel with the planes of the guides while allowing movement perpendicular to the planes of the guides, preferably by being elastically flexible. A stabilizer element of this kind can be realized in a particularly simple manner through the formation of a connector portion similar to a leaf spring. The connector portion extends between the arm and the body of the stationary part with at least one bend in between. The purpose of the bend is to ensure the required flexibility perpendicular to the planes of the guides.
A connector portion of this kind provides a particularly effective stabilization of the arm against moving sideways while maintaining the elastic flexibility in the perpendicular direction, if the connector portionxe2x80x94in a sectional plane that is perpendicular to the pivotal axis of the armxe2x80x94has essentially the shape of an S-curve. Staring at the arm, the connector has a first short segment perpendicular to the guides, bends into a longer segment parallel to the guides, and then bends into a second short segment that connects to the body of the stationary part.
In addition to stiffening the arm against sideways movement, the stabilizer element in the form of a connector portion also helps to prevent torsional deformation of the arm, if the width of the connector portion in the direction parallel to the pivotal axis is significantly greater than its thickness. From a practical point of view, it is best if the connector portion has about the same width as the arm.
To achieve the most compact configuration possible for the inventive force-measuring apparatus, it has proven to be particularly advantageous if, in relation to the lengthwise direction of the arm, the stabilizer element is arranged essentially at a location between the adjustment area and the attachment area. This allows the remaining free space between the guides to be utilized, if the stabilizer element is arranged essentially between the guides, as seen in a sectional plane that is perpendicular to the pivotal axis.
From the point of view of optimizing the construction, it has proven to be particularly advantageous if the stabilizer element is formed in a monolithic unit together with the arm and/or the body of the stationary part, because this arrangement helps to reduce the number of parts required to build the inventive weighing cell.
If one of the guides is connected to the stationary part at two attachment areas that are located at a distance from each other, it is possible to obtain a particularly precise alignment of this guide in relation to the other guide, if the adjuster device allows the positions of the two attachment areas to be adjusted independently of each other in the direction perpendicular to the planes of the guides.
From the point of view of optimizing the construction, it has proven to be particularly advantageous if the adjusting member is provided in the form of an adjustment screw that runs perpendicular to the planes of the guides, passes through a hole in the arm and is engaged in a matching threaded hole of the body of the stationary part. The head of the adjustment screw can push against an outer surface of the arm that faces away from the stationary part, if the adjuster device comprises a pre-tensioning element that urges the arm against the head of the adjustment screw. The attachment area can be a raised pad in the top surface of the arm with a threaded hole for fastening the guide flexure.
In the following, the invention is explained on the basis of the drawing, which is also being explicitly referred to with regard to all details essential to the invention that are not closely portrayed in the description.